Automated guided vehicles are configured to transport parts, products, small objects, and the like at worksites such as factories, warehouses, and offices. Among these automated guided vehicles, there have been one configured to travel along a guide line such as an electric line or light reflective tape, an autonomous travel-type requiring no such guide line, and the like.
Meanwhile, these automated guided vehicles are each equipped with a motor for traveling, a motor-drive main control circuit for supplying power to the motor to operate the motor, and the like.
FIG. 5 is a configuration diagram of a conventional motor-drive main control circuit for an automated guided vehicle. FIG. 5 shows a state where the positive electrode and the negative electrode of a battery are connected normally.
As shown in FIG. 5, a conventional motor-drive main control circuit 1 for an automated guided vehicle includes a battery Batt, a main contactor MC, a motor main control circuit section 2, a battery-positive-electrode-side line L1, a battery-negative-electrode-side line L2, a diode D1, and a voltage smoothing capacitor C1.
The battery Batt is a power source of a DC motor M for travel of an automated guided vehicle and is configured such that a DC voltage of about 12 V to 72 V can be obtained.
The DC motor M is connected to the motor main control circuit section 2. The motor main control circuit section 2 is a circuit configured to be capable of forward and reverse energization of the DC motor M by means of multiple FETs or bipolar transistors so as to realize forward and reverse operation of the DC motor M, and is generally called an H-bridge circuit.
This H-bridge motor main control circuit section 2 is generally configured such that the ON-OFF states (conductive-non-conductive states) of its FETs or bipolar transistors are controlled to perform chopper operation so that the level of the voltage to be applied to the DC motor M can be made variable and therefore the current for energizing the DC motor M and the number of rotations thereof can be made variable.
As a specific example, FIG. 5 shows a configuration of the motor main control circuit section 2 using four Nch FETs T1 to T4. The Nch FET T1 and Nch FET T2 are provided in series, and a source S of the Nch FET T1 and a drain D of the Nch FET T2 are connected at a node 2a. On the other hand, the Nch FET T3 and the Nch FET T4 are provided in series, and a source S of the Nch FET T3 and a drain D of the Nch FET T4 are connected at a node 2b. Moreover, a drain D of the Nch FET T1 and a drain D of the Nch FET T3 are connected at a node 2c, and a source S of the Nch FET T2 and a source S of the Nch FET T4 are connected at a node 2d. 
The voltage to be applied between the source S and a gate G of each of the Nch FETs T1 to T4 is controlled by means of a gate voltage generation circuit (not shown) of each of the Nch FETs T1 to T4 to control the ON-OFF state of each of the Nch FETs T1 to T4 (the conductive-non-conductive state between the source S and the drain D) and thereby perform chopper operation. As a result, the level of the voltage to be applied to the DC motor M is made variable and therefore the current for energizing the DC motor M and the number of rotations thereof are made variable. Moreover, the Nch FETs T1 to T4 include parasitic diodes D11 to D14, respectively. When the energy of the DC motor M is to be regenerated to the battery Batt, these parasitic diodes D11 to D14 function as backward diodes (free-wheel diodes).
The DC motor M is connected at one end M1 to the node 2a of the Nch FETs T1 and T2 and connected at the other end M2 to the node 2b of the Nch FETs T3 and T4.
The main contactor MC is provided on the battery-positive-electrode-side line L1. The battery-positive-electrode-side line L1 connects a positive electrode terminal B1 of the battery Batt and a battery-positive-electrode-side terminal 2e of the motor main control circuit section 2 (i.e. a terminal on the drain D side of the Nch FETs T1 and T3) to each other through the main contactor MC. The battery-negative-electrode-side line L2 connects a negative electrode terminal B2 of the battery Batt and a battery-negative-electrode-side terminal 2f of the motor main control circuit section 2 (i.e. a terminal on the source S side of the Nch FETs T2 and T4).
To bring the automated guided vehicle to an emergency stop, the supply of power from the battery Batt to the DC motor M needs to be securely stopped for safety, and the motor-drive main control circuit 1 is provided with the main contactor MC for this reason. Thus, to bring the automated guided vehicle to an emergency stop, this main contactor MC is opened (contacts MC1 and MC2 of the main contactor MC are opened). In this way, the supply of power from the battery Batt to the DC motor M can be securely stopped.
The diode D1 is provided on the battery-positive-electrode-side line L1 in parallel with the main contactor MC, and is connected at its cathode K to the primary-side contact MC1 of the main contactor MC (i.e. one on the positive electrode terminal B1 side of the battery Batt) and connected at its anode A to the second contact MC2 of the main contactor MC (i.e. one on the battery-positive-electrode-side terminal 2e side of the motor main control circuit section 2).
This diode D1 is a component not used during normal travel of the automated guided vehicle. The main contactor MC is closed as shown in FIG. 5 during normal travel of the automated guided vehicle. To bring this traveling automated guided vehicle to an emergency stop, the main contactor MC is opened as shown in FIG. 6. In this case, the energy of the DC motor M (inductance energy, or mechanical energy resulting from the rotation of the DC motor M) is converted into electric energy. Like 13 shown in FIG. 6, the electric energy is regenerated to the battery Batt through the backward diodes D12 and D13 of the Nch FETs T2 and T3 in the motor main control circuit section 2 (the backward diodes D11 and D14 of the Nch FETs T1 and T4 during reverse rotation of the DC motor M) and the diode D1 in this order.
If the diode D1 is not provided, the above-mentioned energy of the DC motor M is converted into electric charges and accumulated in the capacitor C1. As a result, the voltage of the capacitor C1 rises. If the energy is large, the voltage of the capacitor C1 may be overvoltage, and components forming the motor-drive main control circuit 1 (elements such as the Nch FETs) may be damaged.
As solutions to this problem, measures have heretofore been proposed such as adding a regenerative resistor to a motor-drive main control circuit, yet the simplest configuration is the configuration like the motor-drive main control circuit 1 in which the diode D1 is added.
The voltage smoothing capacitor C1 is connected at one end C1a to the battery-positive-electrode-side line L1 (i.e. the secondary-side contact MC2 of the main contactor MC and the battery-positive-electrode-side terminal 2e of the motor main control circuit section 2) and connected at the other end C1b to the battery-negative-electrode-side line L2 (the negative electrode terminal B2 of the battery Batt and the battery-negative-electrode-side terminal 2f of the motor main control circuit section 2), and thus is provided in parallel with the motor main control circuit section 2. This voltage smoothing capacitor C1 is configured to suppress voltage fluctuations due to fluctuations in the load on the power source (battery Batt) resulting from the chopper operation of the motor main control circuit section 2 (H-bridge circuit).
In the motor-drive main control circuit 1 with the above configuration, when the main contactor MC is closed (the contacts MC1 and MC2 of the main contactor MC are closed) and the Nch FETs T1 and T4 of the motor main control circuit section 2 are turned on, current flows in the forward direction like I1 shown in FIG. 5 and energizes the DC motor M, thereby rotating the DC motor M in the forward direction like R1 shown in FIG. 5. On the other hand, when the Nch FETs T2 and T3 are turned on, current flows in the reverse direction like 12 shown in FIG. 5 and energizes the DC motor M, thereby rotating the DC motor M in the reverse direction like R2 shown in FIG. 5. Moreover, the current for energizing the DC motor M and the number of rotations thereof vary due to the chopper operation of the motor main control circuit section 2 (H-bridge circuit).